Due to the increasing demand for high quality metals, the purification of molten metals is becoming increasingly crucial. As a result, methods for the detection, measurement, separation and removal of inclusions from molten metals are desirable. In particular, the aluminum casting industry is in need of a reliable, fast, and economical detection system that enables furnace operators to conduct fine metal cleaning operations, and thus prevent small defects in the finished products.
A typical aluminum melt, for example, contains a large number of small non-metallic inclusions, less than or equal to 50 xcexcm in size. These include particles of oxides (Al2O3), spinels (MgAl2O4), and carbides (SiC, Al4C3), with a higher melting point. Inclusions in alloys can impair the mechanical properties of articles made therefrom, are also detrimental to surface finish and machinability, increase internal porosity in the castings, as well as increase corrosion. Non-metallic inclusions act as stress-raisers, and can cause premature failure of a component.
The assessment of the level of inclusions present in the melt is one of the key parameters which needs to be measured in molten metal processing. The existing detection techniques include pressure filter test, acoustic emission detection, and electric resistivity Coulter counter. The first two methods mainly rely upon a qualitative distinction between heavily contaminated melts and a clean melt. The Coulter counter method evaluates both concentration and size distribution of inclusions larger than 15-20 xcexcm for a small probe. However, this method is quite expensive and can only detect the effective size of an inclusion.
The present invention relates to a system for detecting and measuring non-metallic inclusions in molten metals. The methods for measuring inclusions in molten metals of the present invention include the steps of forcing the migration of the contaminant particles or inclusions onto a measurement region or surface using electromagnetic Lorentz forces, for example, detecting the particles in the measurement region and determining particle size and concentration at the measurement surface.
Electromagnetic force mechanisms have been investigated and used for purposes of separation and removal of contaminants in liquid metals. However, the cleaning systems relying on electromagnetic forces are not very effective because a very low force density is typically generated in a large liquid metal melt volume which needs to be cleaned, resulting in a slow relative particle motion. In the present invention electromagnetic forces are used to detect and measure non-metallic inclusions in a liquid metal. A detector system uses a small inspection volume, thus allowing for the generation of large force densities. The present invention may also be used to separate inclusions from metals such as aluminum utilizing the basis of high electromagnetic force density in channels having small volumes.
In particular, a preferred embodiment utilizes permanent magnets and a direct current (DC) source to generate electromagnetic forces. In addition, the methods for the detection of inclusions utilize electrostatic detection of the particle concentration at the measurement region or surface through a multi-pin measurement configuration. Further, conditioning of the surface is required to overcome the surface tension forces that are responsible for preventing the inclusions from penetrating through the melt surface. By conditioning the surface, the particles penetrate the surface in order to be detected. The methods of conditioning the surface to enable particle detection can comprise a mechanical system or an acoustical vibration system or a combination of these two systems. A mechanical system can use, for example, a roller, to continuously stretch out the surface layer of the melt. An acoustical vibration system involves the shaking of the liquid melt surface at a particular resonance frequency, for example 10-40 Hz depending on the geometric size of the inspection volume, using an alternating current (AC) superimposed over the DC current flowing through the melt. The surface vibrations stimulate particle motion. Alternatively, a stream of a gas, or mixtures of gases, can be directed over the surface of the melt. Gas pressures in the cavity above the melt can be between 2-3 atmospheres, for example, to condition the surface. The gas flow can be used to delay oxidation and/or reduce surface tension on the melt surface. This serves to increase migration rates of inclusions to the surface region of the melt. Depending upon the direction and rate of flow, one or more gas inlets and outlets to the cavity above the melt can be used to control conditions on the surface region of interest. Inert gases such as helium or argon can be used, or active fluids such as chlorine gas can be used with or replace the inert gas which can also serve to loosen bonds at the surface to further improve particle migrations and detection. These gases can also improve the contrast in the heat signature of surface region components.
In another preferred embodiment of the present invention, the detection system is an optical system which features a solid state imaging device such as a charge-coupled-device (CCD). The CCD based detector system facilitates the electronic recording of the particles distributed over the surface aperture. Once the particles are collected on the measurement surface or free melt surface by the application of electromagnetic Lorentz forces, low-frequency acoustic vibrations are initiated to enable the migration of the particles through the metal melt. Recording of the particle size and distribution is performed with the CCD camera in conjunction with optical magnification of the region of interest using a lens system. The CCD camera may be coupled to an image acquisition system, which in turn may be coupled to a processor such as a microcontroller or personal computer having an electronic memory for data storage. The systems can be programmed with software modules to perform image processing on the collected image data and determine quantitative values including particle size and distribution. This processed data can be used to control flow rates and separation rates of the system.
In another preferred embodiment of the invention, detectors or detector systems sensitive in the range of wavelengths from 500-1200 nm are used to count inclusions. By detecting in the visible, near infrared and infrared regions of the electromagnetic spectrum, subsurface particles can be detected as well. Commercially available detectors, such as amorphous selenium, can be used with a quartz window to image surface and subsurface particles at video frame rates.
Yet another embodiment of the present invention uses only an AC power source to induce electromagnetic forces in the melt and thereby cause movement of the melt and consequent positioning of inclusions for measurement. The detection system can be used in conjunction with a system for the separation of inclusions from the melt and provide real-time feedback control of the processing operation. The systems of the present invention provide for the quantitative measurement of small inclusions, and can determine particle shape. Further, the systems of the present invention can distinguish between a single particle and a cluster of particles, and can distinguish between gas bubbles and solid particles. There are several applications of the systems of the present invention including but not limited to the detection of inclusions in molten metals and the separation of inclusions from molten metals such as aluminum, ferrous materials, brasses and copper based alloys. In addition, the systems of the present invention may be utilized in semi-solid processing or die casting to homogenize segregated interdendritic liquid as well as breaking up dendritic networks.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.